The present invention is related to a capacitor with integral components, preferably, resistors and fuses, wherein failure of the capacitor results in a resistively loaded or protected circuit.
Capacitors are utilized in virtually every electronic circuit and their function is well documented. As with any electronic component capacitors are not immune to failure. Failures can be caused by physical strain, by electrical overloads, or by a myriad of other causes. In cases where capacitor failure is due to an internal short, or an electrical overload, the failure itself can cause damage to the remaining circuit or the failure eliminates protection of the remaining circuit from subsequent events. If capacitor failure results in a direct short, for example, the dampening effect on the remaining circuit is lost resulting in potential un-dampened high loads. Designs have been developed that allow the capacitor to fail in an open mode, thus minimizing the opportunity for a direct short and creating an open in the circuit. This failure mode renders the capacitor non-functional but does protect the circuit from an overload condition. However, certain open mode failures, caused by cracked capacitors as an example, when exposed to voltage, can result in metal migration creating an intermittent open or short over time. It is known that voltage in the presence of humidity results in an increased rate of metal migration accelerating the failure.
Fuses and resistors have been incorporated into capacitors in the past as indicated in U.S. Pat. Nos. 7,165,573 and 4,193,106. Different metallization technologies exist that can be utilized to integrate the use of a resistor or fuse with the capacitor. Each technology has its own advantages and disadvantages which have to be considered in order to make the most advantageous choice. One such technology is referred to in the art as “thin film metallization”. Thin film metallization is based on vapor deposition of materials onto a surface. Circuits are then created by successive steps of masking, imaging, and etching. This process is the mainstay of semi-conductor manufacturing which is a batch operation. It is cost prohibitive when utilized to generate circuits one part at a time.
Thick film technology is also known. Thick film technology is based on materials which are conductive, resistive, or insulative in nature being formed into a paste that is applied to a surface in desired patterns by means of a printer, such as a screen printer. The paste is placed in a particular location to create a desired circuit pattern. Thick film technology can be utilized in different approaches. One is to apply the metallization directly to the surface of the capacitor as taught in U.S. Pat. No. 7,164,573 which is commonly assigned. U.S. Pat. No. 7,164,573 describes formation of a resistor or a fusible link in series with a capacitor on the surface of the capacitor utilizing thick film technology. The current carrying capabilities of the fusible link can be adjusted by varying the length and cross sectional area of the conductive link itself. The resistor value can be adjusted by its cross sectional area and length, the resistivity of the ink itself, or by using a laser to actually trim the resistor.
Implementing the cited prior art is difficult for, at least, two reasons. The first is that printing the required conductive traces as well as the functional components onto the surface of a capacitor requires each capacitor be printed individually, or in multiples, that require special tools and requires the thickness of all the capacitors to be held within a tolerance of +/−0.003 inches in order to minimize part to part conductor thickness variation. This requires an extensive amount of parts handling, tooling and automation. Special tooling is required to properly locate each capacitor at each printing step, thus requiring a high level of capitalization. The second issue is that thick film materials are designed to be fired in an air atmosphere. This is compatible with Precious Metal Electrodes (PME) but not Base Metal Electrodes (BME). PME, typically utilizing palladium and silver, are cost prohibitive in many cases. BME systems, which typically utilize nickel, are dramatically less expensive and are therefore preferred. Unfortunately, BME systems must be fired in a reducing atmosphere in order to keep the nickel from oxidizing. Therefore, the practice of utilizing thick film technology is limited to a small mix of products that utilizes the PME electrode system and cannot be applied to BME electrode systems because of the incompatibility of the air fired material systems. Even in the case of PME systems the thick film materials have to be carefully matched to the capacitor materials for them to function.
For the reasons set forth above, the art lacks a thick film material system that is compatible to both the PME and BME material systems.